Chipps, Michael John; (1998) An experimental investigation into filter ripening: Contact filtration of lowland reservoir water. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Pilot rapid gravity filters, treating lowland reservoir water, were operated to examine chemical, physical and biological influences on filter ripening. Ripening occurred if, after backwashing, filtrate turbidities showed an initial peak, then improvement to a minimum value. Backwashing using collapse pulsing prevented biological fouling of media, and ensured constant starting head losses for three years. Ripening was examined using sand roughing filtration and dual media contact filtration with pre-ozonation and iron (III) sulphate coagulation. Turbidity removal data from sand filters showed that ripening behaviour varied with seasonal changes in applied loading. Biological maturation of filter media over several months caused improved turbidity removals. Subsequently, ripened turbidity removals varied proportionally with water temperature. Within a filter run ripened turbidity removal was independent of load at constant flow rate, but varied inversely with flow rate. Dual media filters showed ripening behaviour in roughing and contact filtration modes. Iron dosing produced superior turbidity removals. A novel coagulation control method was devised, based on the shape of the filtrate turbidity curves. Ripening time varied inversely with coagulant dose, but higher doses caused turbidity breakthrough. A temporary initial overdose could control ripening. Pre-ozonation enhanced roughing and contact filtration, but did not change the time taken to achieve maximum removals. With contact filtration ripened removals were a fixed proportion of the applied turbidity in winter, but filtrate turbidity was independent of load in summer, achieving 0.2 NTU. Flow rates of 12, 17 and 22 m.h⁻¹ produced identical ripening curves, indicating a complex relationship between flow rate and particle capture. This was confirmed with data showing inconsistent relationships between turbidity, particle loading and ripening, and poorer ripening behaviour on longer filter runs. Further detailed particle size data were required. Six definitions of ripening were examined but none was satisfactory as ripening behaviour changed with chemical treatment and seasonal factors.

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